The present invention pertains to the field of coated cutting tools. It is especially concerned with those coated cutting inserts which may be subjected to repeated impacts during use, such as would be encountered during milling or other interrupted cutting operations.
Among the various metalcutting processes, milling is the most demanding operation as far as cutting inserts are concerned. Milling involves interrupted chip removal at constant speed. The tool tip repeatedly enters, cuts and leaves the workpiece, sustaining both mechanical and thermal shocks. The magnitude of these shocks depends on the length of the cut and the interval between cuts. The entry and exit parameters may also determine the extent of the mechanical shock produced in the tool material. These conditions are aggravated when cutting speeds are increased.
Cemented carbide cutting inserts employed in the milling operation often exhibit a number of parallel thermal cracks oriented normal to the cutting edge. These thermal cracks, by themselves, do not limit tool life. However, the inserts may also exhibit cracks parallel to the cutting edge. These cracks are believed to originate from the imposed mechanical shocks. The intersection of the thermal and mechanical cracks can cause edge chipping and premature tool failure.
The prevailing practice to combat the edge chipping problem with a given cutting edge geometry is to design the cemented carbide substrate with higher levels of the cobalt binder. This solution reduces the chipping problem, but introduces another problem, namely, deformation and/or flank wear of the cutting edge.
In the past, cemented carbide grades, both with and without a coating, have been utilized in milling applications. Examples of uncoated milling inserts include K2885 inserts and K2884 inserts, which are tungsten carbide based cemented carbide tools having about 10.5 and 8.5 weight percent cobalt, respectively, and both containing various levels of the solid solution carbide forming elements, tantalum, niobium and titanium. Examples of coated milling inserts include KC710 inserts and KC720 inserts, which are tungsten carbide based cemented carbide tools which have been PVD (physical vapor deposition) coated with a layer of titanium nitride having high residual compressive stresses. The substrates used in KC710 and KC720 tools contain about 8.5 and 11.5 weight percent cobalt, respectively, and various amounts of solid solution carbide forming elements.
It is known that PVD coatings may be applied to cemented carbide substrates by a variety of techniques, such as ion plating, magnetron sputtering and arc evaporation. In addition, each technique has many variations. It has been observed that these techniques and their variations result in PVD coated tools with a variety of properties. Depending on the exact technique used to deposit the coating, properties such as coating hardness, residual stress, tendency to react or bond to the substrate may be positively or adversely affected. These PVD techniques and the properties of the resulting coatings and how PVD coatings compare to CVD (chemical vapor deposition) coatings are described in: Buhl et al, "TiN Coatings on Steel," Thin Solid Films, Vol. 80 (1981) pages 265-270; Buhl et al, U.S. Pat. No. 4,448,802 (foregoing described the Balzers AG ion plating technique and equipment used by the applicants herein); Munz et al, "A High Rate Sputtering Process for the Formation of Hard Friction-Reducing TiN Coatings on Tools," Thin Solid Films, Vol. 96 (1982) pages 79-86; Munz et al U.S. Pat. No. 4,426,267; Kamachi et al, "A Comparison of Residual Stresses in Cemented Carbide Cutting Tips Coated with TiN by the CVD and PVD Processes and Their Effect on Failure Resistance," Surfacing Journal International, Vol. 1, No. 3 (1986) pages 82-86; Wolfe et al, "The Role of Hard Coatings in Carbide Milling Tools," Journal of Vacuum Science Technology, A4 (1986) pages 2747-2754; Quinto et al, "High Temperature Microhardness of Hard Coatings Produced by Physical and Chemical Vapor Deposition," Thin Solid Films, Vol. 153 (1987) pages 19-36; Jindal et al, "Adhesion Measurements of Chemically Vapor Deposited and Physically Vapor Deposited Hard Coatings on WC-Co Substrates," Vol. 54 (1987) pages 361-375; Jindal et al, "Load Dependence of Microhardness of Hard Coatings," Surface and Coatings Technology, Vol. 36 (1988) pages 683-694; Rickerby et al, "Correlation of Process and System Parameters with Structure and Properties of Physically Vapour-Deposited Hard Coatings," Thin Solid Films, Vol. 157 (February 1988) pages 195-222; Quinto et al, "Mechanical Properties, Structure and Performance of Chemically Vapor-Deposited and Physically Vapor-Deposited Coated Carbide Tools," Materials Science and Engineering, A105/106 (1988) pages 443-452.
It is the inventors' opinion that the technique that provides the best PVD coating is that described in the Buhl et al patent and article mentioned above which utilizes the Balzers AG ion plating technology and equipment. This belief is based on their analysis of different types of PVD coated tools which have shown that, in PVD TiN coatings, the highest hardnesses and the highest compressive residual stresses are attainable in the Balzers AG ion plated PVD coating. These properties produce a cutting tool that has higher wear resistance and less susceptibility to edge chipping and breakage than possessed by other PVD coated cutting tools. The PVD TiN coating on KC710 and KC720 is produced by the above Balzers PVD technique.
In addition, binder enriched grades of cutting tools have also been utilized in milling applications. These inserts include KC850 inserts and KC950 inserts. KC850 inserts have a C-type porosity cobalt enriched tungsten carbide based cemented carbide substrate having a bulk cobalt content of about 5.9 weight percent cobalt, and having additions of solid solution carbide forming elements. The KC850 inserts further have a CVD (chemical vapor deposition) coating on the substrate, having three layers: an inner layer of titanium carbide; an intermediate layer of titanium carbonitride and an outer layer of titanium nitride. The CVD three layer coating is described in U.S. Pat. No. 4,035,541.
The KC950 inserts have an A type porosity, cobalt enriched tungsten carbide based cemented carbide substrate having a bulk cobalt content of about 6.0 weight percent and having additions of solid solution carbide forming elements. The KC950 inserts further have a CVD coating composed of an inner layer of titanium carbide, a middle layer of aluminum oxide and an outer layer of titanium nitride. The cobalt-enrichment in KC850 inserts and KC950 inserts occurs in a zone at the periphery of the cemented carbide substrate and may be accompanied by solid solution carbide depletion. The cobalt concentration in the enriched zone typically has a maximum value in the range of 150 to 300 percent of the bulk cobalt content. The enrichment mechanism in KC950 is described in U.S. Pat. No. 4,610,931. (KC950, KC850, KC, KC710, KC720, K, K2885 and K2884 are trademarks of Kennametal Inc. for its cemented carbide grades of cutting inserts and are used as such herein).
CVD coatings are characterized by residual tensile stresses and thermal cracks due to differences in the thermal expansion coefficients of the CVD coating and the cemented carbide substrate. Therefore, CVD coated tools are more susceptible to edge chipping than PVD coated tools.
While the foregoing prior art cutting insert grades have all been commercially useful, there is always a need to further reduce the aforementioned problems associated with milling, of either limited cutting edge lifetimes due to premature chipping or due to deformation and/or flank wear.